Yahoo's email service has been unavailable to some users since Monday night due to a hardware problem that the Internet company said has been harder to fix than it expected. "The issue has been harder to fix than we originally expected," Jeffrey Bonforte, SVP of Communication Products, Yahoo, wrote in a blog post.

Yahoo believes the problem will be fixed by 3 p.m. PST Wednesday.

The email outage began at 10:27 p.m. PST on Monday.

Yahoo Mail is the most popular Web-based email product among desktop computer users in the United States, according to analytics firm comScore.